[MITgcm-support] ETA with making Boussinesq approximations

[Martin Losch]

Hi lian,

the ocean model pressure coordinates is a non-boussinesq model (I  
guess that's what you want).

In pressure coordinates the variables are re-interpreted, e.g. the  
vertical coordinates rc and rf are no longer "z" in meters, but  
pressure "p" in Pascals. wVel is no longer the vertical velocity in m/ 
s but a pseudo velocity omega=dp/dt in Pascal/s. Eta is no longer the  
position of the sea surface, but it is the bottom pressure anomaly (in  
Pascals); if you use the diagnostic ETAN, you find that in the case  
it's called "bottom pressure anomaly" and in the z-coordinate case  
it's called "surface height anomaly". Basically the model is turned  
upside down with k=Nr being the surface and k=1 the bottom of the  
domain (even below the topography). Please have a look at Losch et al  
2004, JPO, or Marshall, J., A. Adcroft, J.-M. Campin, C. Hill, and A.  
White, Atmosphere-ocean modeling exploiting fluid isomorphisms, Mon.  
Wea. Rev., 132, 2882-2894, 2004.Available from: http://mitgcm.org/pdfs/a_o_iso.pdf 
, doi:10.1175/MWR2835.1 and the documentation for more details.

Likewise the bottom pressure anomaly diagnostics 'PHIBOT  ' in z- 
coordinates becomes the surface height anomaly diagnostics in p- 
coordinates, in both cases you need the appropriate scaling constants  
to get the correct units (in p-coordinates it's PHIBOT/gravity, I  
believe).

Martin



On Aug 26, 2009, at 5:44 AM, 连展 wrote:

> Hi,everyone,
>
>     I am trying to study the change of ETA in a model with making  
> Boussinesq approximations.I start from the example experiment  
> "tutorial_global_oce_in_p". However,I find that the model output ETA  
> in pressure coordinates in this experiment.Can I output the ETA in Z- 
> coordinates diretly?
>     I translated the ETA from  pressure coordinates  to Z- 
> coordinates by dividing rho（suferface layer）*g，and I found the  
> result is very different from the result of experiment  
> "tutorial_global_oce_latlon".Does "ETA" have the same meaning in  
> this two experiment?How could I get the free-surface_r-anomaly(m) in  
> a model with making Boussinesq approximations?
>
> Thanks!!
>
> lian
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